Apparatus and methods for measuring formation characteristics

ABSTRACT

An apparatus can measure characteristics of a formation surrounding a borehole. The apparatus includes a tool body having a neutron measurement section and a density measurement section. The neutron measurement section includes a neutron source and a neutron detector arrangement spaced in an axial direction from the neutron source. The density measurement section includes a gamma ray source and a gamma ray detector arrangement spaced in an axial direction from the gamma ray source. The neutron measurement section and the density measurement section are positioned in the tool body so that the sections overlap in the axial direction and are azimuthally spaced apart in the tool body. The tool body also includes shielding to block a direct signal path from the neutron source to the gamma ray detector arrangement and to block a direct signal path from the gamma ray source to the neutron detector arrangement.

This application is a continuation of and claims the benefit of priorityto co-pending U.S. patent application Ser. No. 12/837,450, which wasfiled on Jul. 15, 2010, which claimed the benefit of priority toEuropean Patent Application 09165721 which granted as European PatentNumber 2275840, which was filed on Jul. 16, 2009.

TECHNICAL FIELD

This invention relates to apparatus and methods for measuring propertiesof formations surrounding a borehole. In particular the inventionrelates to apparatus for measuring properties such as density and/orporosity of the formation surrounding boreholes of the type used in theoil and gas industry.

BACKGROUND

A number of techniques and apparatus are known for characterising theproperties of the formation surrounding boreholes. Typically a toolbody, comprising a signal source and sensors, is placed in the boreholeand the source used to interrogate the formation. A signal returning tothe tool body is measured and the parameter of interest calculated.WO96/08733 and U.S. Pat. No. 5,608,215 disclose neutron-neutron andneutron-gamma techniques for determining the density of a formation.

One well known suite of measurements is known as the ‘triple combo’ andis implemented using a measurement tool capable of measuring formationresistivity, porosity and density (and natural gamma ray) properties,and potentially several ancillary measurements.

Typical triple combo measurements arrange the devices for takingmeasurements in a vertical sequence along the tool body. In many cases aresistivity measurement device (induction, propagation, laterolog etc)is located at the bottom of the tool string, with a neutron measurementdevice following above. A natural gamma ray detector of the tool islocated in such a way that it is not influenced by activation from aleading neutron source. One example of a known wireline triple combotool is from Schlumberger and comprises a Dual Induction Tool (DIT)measuring resistivity, a Litho Density Tool (LDT) measuring gamma raydensity, and a Compensated Neutron Tool (CNT) measuring porosity.

Locating the different sections down the tool increases the length ofthe tool. Reducing the length of the tool reduces the distance betweenmeasurements which helps in the interpretation of results and alsoreduces the time for rig up and rig down, and can help reduce the riskof the tool getting stuck in the wellbore. One example of a shorterwireline tool with similar capabilities is the Platform Express tool ofSchlumberger which utilises integration of the structure of the variousfunctional sections to achieve shorter overall length. Platform Expresstools comprise an Array Induction Tool (AIT), a Three Detector LithologyDensity tool (TLD), a Highly Integrated Gamma Ray Neutron Sonde (HGNS).A Micro Cylindrically Focused Log (MCFL) for shallow resistivity isco-located with a High Resolution Resistivity Gamma Detector (HRGD) fordensity measurement in a pad.

U.S. Pat. No. 7,073,378 describes a tool having a resistivitymeasurement device comprising a multiplicity of antennae interleavedwith a neutron measurement device to reduce the length of the tool.

In logging while drilling (LWD) tools certain measurement sections havebeen co-located in a single collar, for example, the Compensated DensityNeutron (CDN) tool and Azimuthal Density Neutron (ADN) tool ofSchlumberger.

The axial separation of the different measurement sections means thatthe investigation of all the formation properties cannot take placeduring the same time interval. This means that there can be changes inthe formation properties (invasion, damage), the borehole (mud weight,borehole size, rugosity) or the tool position in the borehole (standoff)between the multiple measurements.

This invention aims to provide techniques for locating the variousmeasurements closer together such that the interpretation of themeasurements can be simpler and more accurate.

SUMMARY

This invention provides an apparatus for measuring characteristics of aformation surrounding a borehole comprising:

a tool body including:

a neutron measurement section including a neutron source and a neutrondetector arrangement spaced in an axial direction from the neutronsource; and

a density measurement section including a gamma ray source and a gammaray detector arrangement spaced in an axial direction from the gamma raysource;

wherein the neutron measurement section and the density measurementsection are positioned in the tool body so that the sections overlap inthe axial direction and are azimuthally spaced apart in the tool body;and wherein the tool body also includes shielding to block a directsignal path from the neutron source to the gamma ray detectorarrangement and to block a direct signal path from the gamma ray sourceto the neutron detector arrangement. The term gamma-ray includes x-raysand comprises any photons with energy >1 keV.

In one embodiment, the neutron measurement section and densitymeasurement section are located on opposite sides of the tool body. Inanother embodiment, the neutron measurement section and the gamma raymeasurement section are located to one side of the centre line of thetool. In this case, the direct line path between the two measurementsections does not pass through the tool centre line. Where the tool bodyincludes a channel for flow of drilling fluid, the measurement sectionscan be positioned so that the straight line path does not pass throughthe channel

The shielding can comprise neutron shielding located adjacent theneutron source to block a direct neutron path to the gamma ray detectorarrangement and gamma ray shielding located adjacent the gamma-raysource to block a direct gamma ray path to the neutron detectorarrangement.

The gamma ray detection arrangement can be provided with shielding toreduce neutron-induced background. In addition, to the neutron detectors(thermal, epithermal or fast neutron), the neutron detection arrangementcan comprise one or more gamma ray detectors provided with shielding,and optionally, focusing structures to reduce direct and indirectinterference from the gamma ray section.

The neutron section can comprise shielding to block direct neutrontransmission from the source to the neutron detector arrangement, andthe gamma ray section can comprise shielding to block direct gamma raytransmission from the source to the gamma ray detector arrangement

The neutron measurement section can comprise a plurality of gamma rayand neutron detectors, e.g. near and far gamma ray detectors and nearand far spaced neutron detectors. The gamma ray measurement section cancomprise a plurality of gamma-ray detectors, traditionally a long spacedgamma ray detector and a short spaced gamma ray detector.

In one embodiment the neutron source and the gamma ray source arelocated at substantially the same axial position in the tool body. Wherethe neutron source and the gamma ray source are axially spaced apart onthe tool body, the gamma ray source can be axially closer to the gammaray detector arrangement than the neutron source.

The neutron detector arrangement and the gamma ray detector arrangementcan both extend in the same axial direction from their respectivesources in one embodiment. In another embodiment, the neutron detectorarrangement extends away from the neutron source in the opposite axialdirection compared to that of the gamma ray detector arrangement fromthe gamma ray source.

The gamma ray source can comprise an x-ray source.

The neutron source can comprise a pulsed neutron source and the gammaray or x-ray source can comprise a pulsed source.

Preferably the apparatus is mounted in a wire line logging tool or alogging while drilling tool.

Galvanic and/or ultrasonic sensors can be mounted along the same axialextent of the tool body as the neutron and gamma ray sections.

Where the tool body is a logging while drilling tool comprisingstabiliser blades disposed around the body at the same axial position,electronics and power sources can be incorporated into one or moreblades or in the chassis or collar below or adjacent to the blades

A second aspect of the invention comprises a method for measuringcharacteristics of a formation surrounding a borehole using an apparatusaccording to the first aspect of the invention, comprising:

emitting neutrons from the neutron source into the formation;

emitting gamma rays from the gamma ray source into the formation;

detecting neutrons and gamma rays returning to the neutron section fromthe formation resulting from irradiation of the formation with neutrons;

detecting gamma rays returning to the gamma ray section from theformation resulting from the irradiation of the formation with gammarays; and

analyzing the detected gamma rays and neutrons to provide an indicationof the characteristics of the formation.

Where the apparatus comprises a pulsed neutron source the method cancomprise synchronizing the gamma ray measurement in the gamma raysection with operation of the pulsed neutron source. In one embodiment,this can comprise measuring the difference in the gamma ray measurementsbetween the on and off periods of the pulsed neutron source. In thiscase, the measurements when the pulsed neutron source is one can be usedto determine a neutron-induced background. In another embodiment, gammaray measurement can be suspended when the pulsed neutron source is on.In a further embodiment, gamma ray measurements are time binned so thatthose measurements taken when the pulsed neutron source is on can beidentified.

When the apparatus comprises a pulsed neutron generator and a pulsedx-ray source the method can further comprise disabling the x-ray sourceduring the pulsing cycle of the pulsed neutron source.

In another embodiment, the method can comprise operating the pulsedx-ray source only when the pulsed neutron source is off.

Further aspects of the invention will be apparent from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of a first embodiment of the invention;

FIG. 2 shows a cross sectional view of FIG. 1;

FIG. 3 shows a schematic drawing of a second embodiment of theinvention;

FIG. 4 shows a cross sectional view of FIG. 3;

FIG. 5 shows a schematic drawing of a third embodiment of the invention;

FIG. 6 shows a cross sectional view of FIG. 5;

FIG. 7 shows a schematic drawing a fourth embodiment of the invention;

FIG. 8 shows a cross sectional view of FIG. 7;

FIG. 9 shows an example of a drilling rig platform for implementing anembodiment of the invention; and

FIG. 10 shows a LWD tool for implementing an embodiment of theinvention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show a representation of a borehole tool according to theinvention. The tool 10 can form part of a drill string (not shown). Thetool 10 comprises a collar 12 surrounding a chassis 14 that defines amud channel 16. Stabilizer blades 13 may be mounted on the collar. Thetool 10 comprises a neutron measurement section 18 and density (gammaray) measurement section 20 mounted in the collar 12 under thestabilizer blades 13 on opposite sides of the tool. The neutronmeasurement section and density measurement section are azimuthallyspaced apart on the tool body in the same axial plane, such that the twomeasurement sections overlap each other in the axial direction.

The neutron measurement section 18 comprises a neutron source 22, a pairof spaced apart neutron detectors comprising a far spaced neutrondetector 24 and a near spaced neutron detector 26 and a pair of spacedapart gamma ray detectors, comprising a near spaced gamma detector 28and a far spaced gamma detector 30. The neutron source 22 producesneutrons that are emitted towards the formation and are scattered backto the near and far neutron detectors 24, 26 mounted in the collar. Thenear and far spaced gamma detectors 28, 30 measure the gamma raysresulting from the neutron irradiation of the formation. The results ofthe measurements from the neutron detectors can be used to determine theneutron porosity of the formation using known methods.

The neutron source may be a chemical neutron source or a neutrongenerator source. An electronic neutron source can be based on d-T, d-d,t-t sources, i.e. a d-T 14 MeV source. Typically an electronic neutronsource will be pulsed. Typical chemical neutron sources are ²⁴¹AmBe and²⁵²Cf. It is preferable to use ³He detectors as the neutron detectors,however other suitable detectors can also be used.

A neutron monitor detector (not shown) may be installed close to theelectronic neutron source to measure the instantaneous and/or averageneutron output.

The density measurement section 20 comprises a gamma ray source 32 andspaced gamma ray detectors 34, 36. Gamma rays are emitted from the gammaray source towards the formation. The gamma rays are scattered back tothe gamma detectors and detected by short spaced 34 and long spaced 36detectors mounted in the collar of the tool. The gamma ray source andthe neutron source are axially aligned and azimuthally spaced 180°apart.

A typical element for a chemical gamma ray source may be ¹³⁷Cs. Thedetectors for the gamma rays may be scintillation detectors (NaI(T1),LaBr₃, GSO etc) coupled to ruggedized photomultipliers or otheramplification devices.

Shielding material 38 is provided in the collar 12 and chassis 14 toreduce the level of crosstalk between the two measurement sections 18,20.

The axial overlap of the neutron detector device 18 and the gamma raydetector device 20 allows a tool to be achieved that is substantiallyshorter than those known from the prior art.

The shielding material 38 prevents the neutrons generated from theneutron source 22 and the gamma rays generated from the gamma ray source32 from directly reaching the detectors 24, 26, 34, 36. Source shielding38 is provided around the gamma ray and neutron sources 32, 22 to blockdirect irradiation of the detectors on the other side of the tool. Backshields 40 can be provided around the density detectors 34, 36, toshield the detectors from the impact of neutrons to reduce the neutroninduced background. Additional measures may help to subtract the neutroninduced background from the density signal. The gamma ray detectors 28,30 in the neutron section may also have additional shielding 42 andfocusing to reduce the background induced by the direct and indirectgamma rays from the density section.

By having the neutron and gamma ray measurement devices located closetogether in the same axial location, the interpretation of measurementsis simpler and more accurate. In particular the collocation of themeasurement devices allows the investigation of all the formationsproperties during the same time interval. This means that there islittle or no change in the formation properties (i.e. invasion, damage),the borehole (i.e. mud weight, borehole size, rugosity) or the toolposition in the borehole (standoff) between the multiple measurementsbeing made.

Although the invention is exemplified using a gamma ray source todetermine the density of the formation, the gamma ray source could bereplaced with an x-ray source. Where an electronic x-ray source is usedthis will allow the pulsing of x-rays when irradiating the formation.

It is not necessary that the two sources are precisely axially aligned.As shown in FIGS. 3 and 4 the tool comprises co-located neutron anddensity measurement sections with axially space apart neutron and gammasources. The tool 40 comprises neutron measurement section 18 and thedensity measurement section 20 located on opposite sides of the collar12. Compared to the embodiment in FIG. 1 the gamma ray source 32 andgamma detectors 34, 36 are shifted down the tool, such that the gammasource is no longer axially in line with the neutron source 22. Thedensity measurement section 20 and the neutron measurement section 18are still located in the same area of the tool body 10 and as such thetwo sections overlap each other. Positioning the gamma ray source sothat it is not axially in line with the neutron source can help reducethe influence of the neutron radiation and the neutron induced gammarays on the density measurements, compared to the embodiment where thegamma and neutron source are located at the same axial position on thetool.

It is not necessary that the two measurement sections are located onopposite sides of the collar. As shown in FIGS. 5 and 6 the measurementsections 18, 20 can be located at different azimuths. In this embodimentof the invention the neutron measurement section 18 and the densitymeasurement section 20 are collocated at different azimuths about thetool body 50. The tool comprises the neutron measurement section 18 andthe density measurement section 20 located axially in the same positionbut are located at different azimuths (both to one side of the toolcentre line) about the tool compared to the embodiment of FIG. 1. Bylocating the measurements in the tool body such that they are notlocated opposite each other, the mud passage 16 can not provide such aneasy passage for neutrons and gamma rays to pass directly from onemeasurement section to the other.

In a further embodiment of the invention the measurement sections of thetool may be partially co-located as shown in FIGS. 7 and 8. The neutron22 and gamma ray 32 sources of the measurement sections are located atsubstantially similar axial positions on opposite sides of the tool body60 however the detecting sections are at opposing axial positions, sothat only part of each measurement section overlaps the othermeasurement section.

Stabilizer blades may be mounted on the outer periphery of the tool andcan provide improved contact between the tool and the formation.Electronics and power sources can be mounted into one or more blades.The PNG source comprises a high voltage supply, as can the pulsed x-raysource. These high voltage supplies can be incorporated into a separatestabilizer blade mounted on the tool to further help in minimising thelength of the tool. The electronics and power sources can also beincorporated into the chassis or collar below or adjacent to the blades.

The tool may further include galvanic sensors. The galvanic sensors canbe used to measure the resistivity of the formation and may be mountedon one or more of the stabilizers blades of the tool. Such galvanicsensors measure resistivity by applying a voltage differential acrossparts of the tool which will cause currents to flow from the tool andinto the formation.

Propagation resistivity antennae may be mounted on the tool to overlaythe neutron and/or gamma-ray measurement sections. Having theresistivity antennae overlap at least part of the neutron measurementsection helps reduce the total length of the tool and may also allowsimultaneous measurement of the formation using the neutron measurementdevice and the resistivity measurement device.

Additional sensors can be added to the collar without increasing itslength. These include ultrasonic sensors to give a measurement of thetool stand off. Ultrasonic measurements are well known in the industryas a means to determine the tool stand off from the borehole wall. Thetool stand off is determined by emitting an ultrasonic pulse from thetool and determining the time delay between the emission and thedetection of the reflected signal (echo) in the tool. If the propagationvelocity in the mud is known the tool stand off from the formation canbe determined.

The apparatus can be incorporated into logging while drilling tools orwireline logging tools. For a logging while drilling operation theapparatus is attached to the drilling string to form a bottom holedrilling assembly between the drill string and a drill bit. Other LWDtools may also be attached to the drill string. As the drill string andthe bottom hole assembly rotate, the drill bit bores a borehole throughthe formation. The density and neutron measurements at differentazimuths can be obtained as the drill string and tool of the inventionmove through the formation.

Co-location of the density and neutron measurement sections on the toolcan result in crosstalk. However, the amount of crosstalk can be reducedand/or better quantified where one or both of the sources are pulsed. Byco-locating the measurement sections, it is not necessary to compensatefor the different locations of the sources.

Where a pulsed neutron measurement is used with a chemical source(¹³⁷Cs) based density measurement the pulsed operation can be used tosynchronize the measurements in time.

It is possible to measure the difference in the density count ratesbetween the on and off periods of the pulse neutron generator (PNG),during the burst, during the decay of the capture gamma signal andduring the burst off phase, by synchronizing the density acquisitionwith the neutron pulsing and acquiring the count rates for differenttime intervals with respect to the neutron timing. If the crosstalkduring the neutron burst is too severe, the density measurement could beblocked during the burst period. The loss of duty factor may be morethan compensated by the more accurate and precise backgroundsubtraction. Alternatively the density signal can be binned for thedifferent PNG time gates and corrected separately. This makes itpossible to assess the accuracy of the background subtraction and toexclude time bins from the measurement if necessary.

Where both the neutron source and the x-ray source are pulsed it ispossible to measure the influence of the neutron induced background inthe neutron measurement directly by using a well coordinated pulsingscheme. In this situation the x-ray source can be disabled periodicallyduring an entire PNG pulsing cycle. During this time only the crosstalksignal from the neutron measurement is acquired. This determinedbackground can be subtracted from the density window count rates. Fastx-ray pulsing can done in such a way to acquire the density signal onlywhile the neutron generator is not emitting neutrons, alternatively thedensity acquisition can be gated to be turned off during the neutronburst.

FIG. 9 illustrates a wellsite system in which the present invention canbe employed. The wellsite can be onshore or offshore. In this exemplarysystem, a borehole 11 is formed in subsurface formations by rotarydrilling in a manner that is well known. Embodiments of the inventioncan also use directional drilling, as will be described hereinafter.

A drill string 112 is suspended within the borehole 11 and has a bottomhole assembly 100 which includes a drill bit 105 at its lower end. Thesurface system includes platform and derrick assembly 110 positionedover the borehole 11, the assembly 110 including a rotary table 116,kelly 17, hook 118 and rotary swivel 19. The drill string 112 is rotatedby the rotary table 116, energized by means not shown, which engages thekelly 17 at the upper end of the drill string. The drill string 112 issuspended from a hook 118, attached to a traveling block (also notshown), through the kelly 17 and a rotary swivel 19 which permitsrotation of the drill string relative to the hook. As is well known, atop drive system could alternatively be used.

In the example of this embodiment, the surface system further includesdrilling fluid or mud 26 stored in a pit 27 formed at the well site. Apump 29 delivers the drilling fluid 26 to the interior of the drillstring 112 via a port in the swivel 19, causing the drilling fluid toflow downwardly through the drill string 112 as indicated by thedirectional arrow 8. The drilling fluid exits the drill string 112 viaports in the drill bit 105, and then circulates upwardly through theannulus region between the outside of the drill string and the wall ofthe borehole, as indicated by the directional arrows 9. In this wellknown manner, the drilling fluid lubricates the drill bit 105 andcarries formation cuttings up to the surface as it is returned to thepit 27 for recirculation.

The bottom hole assembly 100 of the illustrated embodiment comprises alogging-while-drilling (LWD) module 120, a measuring-while-drilling(MWD) module 130, a roto-steerable system and motor 150, and drill bit105.

The LWD module 120 is housed in a special type of drill collar, as isknown in the art, and can contain one or a plurality of known types oflogging tools. It will also be understood that more than one LWD and/orMWD module can be employed, e.g. as represented at 120A. (References,throughout, to a module at the position of 120 can alternatively mean amodule at the position of 120A as well.) The LWD module includescapabilities for measuring, processing, and storing information, as wellas for communicating with the surface equipment. In the presentembodiment, the LWD module includes a nuclear measuring device.

The MWD module 130 is also housed in a special type of drill collar, asis known in the art, and can contain one or more devices for measuringcharacteristics of the drill string and drill bit. The MWD tool furtherincludes an apparatus (not shown) for generating electrical power to thedownhole system. This may typically include a mud turbine generatorpowered by the flow of the drilling fluid, it being understood thatother power and/or battery systems may be employed. In the presentembodiment, the MWD module includes one or more of the following typesof measuring devices: a weight-on-bit measuring device, a torquemeasuring device, a vibration measuring device, a shock measuringdevice, a stick slip measuring device, a direction measuring device, andan inclination measuring device.

FIG. 10 shows a logging while drilling device as disclosed in thisapplication, which uses an accelerator-based source, it being understoodthat other types of LWD tools can be used as the LWD tool 120 or part ofan LWD suite 120A. In FIG. 10 a drill collar section 1040 withstabilizer 1042 is shown as surrounding a tool chassis 1054 traversed bya mud channel 1070 for conveying the drilling fluid downward through thedrill string. The stabilizer on the left incorporates a gamma-gammadensity measurement consisting of source 1090, shielding 1092, detectorwindows 1094, detectors 1096 and detector shielding 1098. The stabilizeron the right incorporates an accelerator-based neutron measurement asdisclosed in this application. The measurement consists of a neutrongenerator 1058 with the neutron emitting target 1059, neutron-gammashielding 1072, neutron detectors 1074 and gamma-ray detectors 1076. Aneutron monitor 1070 is used to normalize the output of other detectorsfor source strength. The measurement can be used to determine formationporosity, density, lithology and sigma. U.S. Pat. No. 7,334,465incorporated herein by reference, can be referred to for furtherreference. Shielding 1078 is provided between the two measurements toreduce or eliminate crosstalk between the measurements.

Further embodiments within the scope of the invention will be apparent.

The invention claimed is:
 1. An apparatus for measuring characteristicsof a formation surrounding a borehole comprising a tool body including:a neutron measurement section including a neutron source and a neutrondetector arrangement spaced in an axial direction from the neutronsource; a density measurement section including a gamma ray source and agamma ray detector arrangement spaced in an axial direction from thegamma ray source, wherein the neutron measurement section and thedensity measurement section are positioned to be azimuthally spacedapart, so that each of the neutron source and the neutron detectorarrangement are azimuthally spaced apart from each of the gamma raysource and gamma ray detector arrangement, and overlapping in the axialdirection; and a shielding configured to block a direct signal path fromthe neutron source to the gamma ray detector arrangement and to block adirect signal path from the gamma ray source to the neutron detectorarrangement.
 2. Apparatus as claimed in claim 1, wherein the neutronmeasurement section and density measurement section are located onopposite sides of the apparatus.
 3. Apparatus as claimed in claim 1,wherein the neutron measurement section and the gamma ray measurementsection are located to one side of a center line of the apparatus. 4.Apparatus as claimed in claim 1, wherein the shielding comprises:neutron shielding disposed adjacent the neutron source to block a directneutron path to the gamma ray detector arrangement; and gamma rayshielding located adjacent the neutron source to block a direct gammaray path to the neutron detector arrangement.
 5. Apparatus as claimed inclaim 1, wherein the gamma ray detection arrangement is provided withshielding to reduce neutron-induced background.
 6. Apparatus as claimedin claim 1, wherein the neutron detection arrangement comprises one ormore gamma ray detectors provided with shielding and, optionally,focusing structures to reduce direct and indirect interference from thegamma ray section.
 7. Apparatus as claimed in claim 1, wherein theneutron measurement section comprises shielding to block direct neutrontransmission from the neutron source to the neutron detectorarrangement, and the density measurement section comprises shielding toblock direct gamma ray transmission from the gamma ray source to thegamma ray detector arrangement.
 8. Apparatus as claimed in claim 1,wherein the neutron source and the gamma ray source are located atsubstantially the same axial position in the apparatus.
 9. Apparatus asclaimed in claim 1, wherein the neutron source and the gamma ray sourceare axially spaced apart on the tool body, the gamma ray source beingaxially closer to the gamma ray detector arrangement than the neutronsource.
 10. Apparatus as claimed in claim 1, wherein the neutrondetector arrangement and the gamma ray detector arrangement both extendin the same axial direction from their respective sources.
 11. Apparatusas claimed in claim 1, wherein the neutron detector arrangement extendsaway from the neutron source in the opposite axial direction compared tothat of the gamma ray detector arrangement from the gamma ray source.12. Apparatus as claimed in claim 1, further comprising galvanic and/orultrasonic sensors mounted along the same axial extent of the tool bodyas the neutron and gamma ray sections.
 13. A method for measuringcharacteristics of a formation surrounding a borehole using a tool,wherein the method comprises: emitting neutrons from a neutron sourceinto the formation, the neutron source being part of a neutronmeasurement section; emitting gamma rays from a gamma ray source intothe formation, the gamma ray source being part of a density measurementsection, wherein the neutron measurement section and the densitymeasurement section are positioned to be azimuthally spaced apart, sothat each of the neutron source and the neutron detector arrangement areazimuthally spaced apart from each of the gamma ray source and gamma raydetector arrangement, and overlapping in the axial direction; detecting,at a neutron detector arrangement of the neutron measurement section ofthe tool, neutrons and gamma rays from the formation resulting fromirradiation of the formation with neutrons; detecting, at a gamma raydetector arrangement of the density measurement section of the tool,gamma rays from the formation resulting from the irradiation of theformation with gamma rays; and analyzing the detected gamma rays andneutrons to provide an indication of the characteristics of theformation.
 14. The method as claimed in claim 13, wherein emittingneutrons from a neutron source comprises using a pulsed neutrongenerator.
 15. The method as claimed in claim 13, comprising:synchronizing detection of the gamma ray in the density measurementsection with emission of neutrons from the neutron source.